I've owned and played with LB-7s from the first ones made (serialed under #400) to some of the last made, and there was not a lot of difference in fit and finish.
The number of pins was just a production change to simplify assembly. The 4th pin was a cover pin. It was removed from all knives in the pattern and then added back on the smooth Delrin knives because the delrin covers weren't held tight enough with only the three. That is why you will find the first scrims with four, then some with three, then the later ones with four. Evidenly that wasn't a problem with the more dimensionaly stable phenolic resin impregnated wood covers used on the production LB-7 since they stayed three pin from the change til the end.
As thawk suggested, since four pin indicates a generally older knife, some people value them more highly, particularly for collecting.
Michael